Usually, in a rubber composition used in a tire, process oils have been used as a softener in order to soften the rubber. Examples of the process oil include petroleum softeners such as paraffin process oils, aromatic process oils and naphthene process oils (see JP-A-Hei7-118455).
However, since the process oil migrates through the tire rubber over time and gradually separates out on to the surface to disperse on a road or in the atmosphere, the effect of the softener decreases, and the hardness of the rubber increases.
Particularly in a studless tire, if the rubber hardness increases, there arises the problem that excellent performances on snow and ice cannot be maintained.
Rubber compositions for treads in truck or bus pneumatic winter tires, particularly studless tires for a truck or bus, employ a rubber such as a butadiene rubber that is mixed with a rubber having an excellent tensile strength and abrasion resistance, such as a natural rubber in order to improve ice performance and snow performance by softening the surface. These compositions further employ fillers such as carbon black in order to improve reinforcement and strength. Abrasion resistance is especially desired for truck and bus tires, and there may be some hesitation to compound in a plasticizer for processability and a tackifier for imparting adhesion.
However, if the plasticizer and the tackifier are not compounded, it is likely that processability at molding is insufficient such that problems during processing occur. Particularly, butadiene rubber generally has poor processability and low adhesion properties, which need to be improved. If a low polar plasticizer such as mineral oil is added, processability is improved and adhesion increases, but the abrasion resistance becomes low. It is possible to use a tackifier such as a petroleum resin and a phenol resin, but abrasion resistance inevitably remains low. It is also possible to use an aroma oil. In this case, the abrasion resistance is slightly better compared with results using mineral oil or a tackifier, but it decreases at some point and performance on ice and snow deteriorate.
The demand for improved abrasion resistance in a tire for a truck or bus is high and there have been some instances of winter tire use roads without snow or ice road as a reaction to recent global warming warnings or the inconvenience of changing winter tires for the summer. Therefore, improvement of abrasion resistance in the tread of truck or bus pneumatic winter tires, particularly studless tires for a truck or bus, has been in high demand.